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Network Working Group                                      J. Hildebrand
Internet-Draft                                       Cisco Systems, Inc.
Intended status: Informational                               H. Flanagan
Expires: December 8, 2014                                     RFC Editor
                                                            June 6, 2014

         HyperText Markup Language Request For Comments Format


   This document defines the HTML format that should be used for the
   production of Internet-Drafts and RFCs.  The HTML output will include
   a default CSS to enable page layout, and the HTML itself includes
   semantic information only.  This format will be rendered from the
   canonical XML format for an RFC.

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at http://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on December 8, 2014.

Copyright Notice

   Copyright (c) 2014 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of

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   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

1.  Introduction

1.1.  Background

   The RFC Series has been in existence for over 40 years.  During much
   of that time, the limitations of character set, line and page length,
   and graphics restrictions of RFC documents met the most immediate
   needs of the majority of authors and readers.  As technology changed,
   new formats that allowed for a richer set of edit, search and display
   features came in to use, and tools were created to convert the plain
   ASCII documents to other desired formats such as HTML, PDF, and
   Microsoft Word.  While the converted versions of the RFCs are widely
   available, the canonical display format remains the plain text,
   ASCII, line-printer structured one.

   In 2013, after a great deal of community discussion, the decision was
   made to shift from the plain text, ASCII-only canonical format to XML
   [I-D.hoffman-xml2rfc].  Several different publication formats will be
   rendered from that canonical XML, including HTML, PDF, TXT, and EPUB.

   This memo describes an HTML format that will be used as one of the
   publication formats for the RFC Series.  It defines a strict subset
   of HTML appropriate for Internet-Draft and RFC Series documents, and
   serves as a comprehensive example of all of the HTML elements that
   are permissible.  The CSS that defines the visual layout, while
   included in the HTML file, will be described in a separate document
   [W3C.REC-CSS2-20110607].  The HTML itself will represent semantic
   information only.

1.2.  Terminology

   In this document, the key words "MUST", "MUST NOT", "REQUIRED",
   and "OPTIONAL" are to be interpreted as described in BCP 14, RFC 2119

2.  Requirements for HTML

   The HTML has to render correctly on the following:

   o  the latest released versions of Chrome, Firefox, and IE running on
      Windows 8 in November 2013

   o  the latest released versions of Chrome, Firefox, and Safari
      running on Mac OS X 10.9 in November 2013

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   o  the latest released versions of Chrome and Safari running on iOS 7
      in November 2013

   o  the latest released versions of Chrome and Firefox running on
      Ubuntu 13.10 in November 2013

   o  the latest released versions of Chrome and Firefox running on
      Android 4.1 in November 2013

   These requirements are expected to change in the future to reflect
   the expectation that HTML rendering will be required for current
   versions of browsers and platforms, while ideally continuing to
   render correctly on earlier versions.

   The HTML document must preserve all semantic information that is in
   the canonical XML document.  One use case is that preformatted text
   that has different tags in the XML will also be differentiable in the
   HTML, making it trivial to extract all of the (for example) ABNF in
   an RFC with a simple program.  Another use case is that someone who
   wants to write programs that will extract information from an RFC can
   do so equally well with the XML and HTML, and can choose the tool
   that uses one or the other format for input.

   The HTML document must come with a default, internal set of CSS
   formatting.  This will allow for a mostly-consistent display of RFCs
   across browsers.  It will also allow for the HTML file to be moved
   over different transports (such as e-mail) and have the result look
   the same.

   The HTML must display adequately in at least one text-based browser.
   Any use of javascript must not negatively impact the ability to read
   the document.

   The HTML document must allow easy local override of the default CSS
   formatting.  This will allow users who have a different visual style
   that they prefer to make RFCs display with that style without having
   to alter the contents of the HTML document.  This might also be
   valuable for allowing people with specific accessibility needs to
   have custom CSS.

   No HTML tags in the document may have style information.  All style
   information must be done through "class" and "id" attributes, with
   the style for those represented in the CSS alone.  Exceptions can be
   made for formatting that is not possible in any other way in HTML5
   [W3C.CR-html5-20130806], such as table column widths.

   The HTML must make it easy to separate chunks into separate files.
   This will make creating EPUB documents easier in the future.

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   The output needs to be HTML5.  Language extensions might be
   acceptable after further discussion.  The RFC Editor will use an
   automated validating tool before publishing the HTML.  This
   requirement is not important for viewing with browsers, but is
   important for programs that will use the HTML format as input for

   The HTML must not have any Javascript or other active code in
   <script> or <object> tags.

   All section, subsections, figures, and paragraphs should have stable
   numbered link anchors.  Additionally, anchors expressed in the source
   XML should be exposed as anchors in the HTML as well.

   The abstract must be marked up or tagged in a way that search engines
   will extract it as summary.

   Normative information must be easily accessible to the following

   o  People with impaired vision, including those that use large fonts
      and those that use screen readers

   o  People with difficult;y distinguishing between colors

   o  People who use devices with small screens, such as cell phones

   o  Other groups TBD

   Specific instances where these goals are important in the design
   choices of the format have been called out in the text.

   The HTML document does not require the inclusion of non-semantic
   information such as comments and processor instructions.

   NOTE: designing for these consumers does not preclude the use of
   features they cannot use, but does require that key semantic data is
   not lost when read using the tools and settings that are required by
   a given constituency.

3.  HTML Format

   The format specified here is a subset of HTML, deemed to be widely-
   implemented by common browsers at the time that the specification was
   created, likely to continue to be widely-implemented in the future,
   and unlikely to cause security issues.

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3.1.  Syntax

   The following rules SHALL be enforced:

   o  The HTML source MUST be encoded as UTF-8, as specified in RFC3629.
      Note that RFC3629 forbids "surrogate" codepoints in the range
      U+D800 to U+DFFF.

   o  The document MUST be valid HTML5.

   o  Single quotes (U+0027 APOSTROPHE: ') MUST be used to quote
      attribute values.  Unquoted attribute values MUST NOT be used.

   o  Each logical line MUST be terminated solely with a \n (U+000A:
      LINE FEED), otherwise known as "Unix-style" line endings.

   o  Other than \n (U+000A: LINE FEED), code points less than " "
      (U+0020: SPACE) (otherwise known as "control characters") MUST NOT
      be used.  Any character references that would generate these code
      points (e.g. ) MUST NOT be used.  NOTE: this rule explicitly
      forbids \t (U+0009: CHARACTER TABULATION), \f (U+000C: FORM FEED),
      and \r (U+000D: CARRIAGE RETURN) from appearing in the source.

   o  Each text-containing element such as headings (<h1>-<h6>),
      paragraphs (<p>), or list items (<li>), MUST be serialized as a
      single line without wrapping.

   o  The contents of <pre> elements MUST NOT be modified by processing
      tools.  The following rules apply to all elements except for

      *  HTML SHALL be indented using spaces (not tabs).

      *  Each child element SHALL be indented two spaces more than its
         parent element, unless the child element is mixed with non-
         whitespace-only text children of the same parent element.

   NOTE: none of these rules affect the rendered output of the HTML, but
   are intended to increase the chance that multiple tools that process
   the format will generate identical syntax.  In turn, this will make
   difference tools that operate on the HTML source easier to write.

3.2.  Basic Structure

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3.2.1.  HTML5

   The HTML comprising the document MUST be valid according to the
   latest version of the HTML specification at the time of publishing,
   starting with the version commonly known as HTML5.  Although the HTML
   specification mandates several of syntax and structure rules in this
   document, they are called out here for emphasis.

3.2.2.  DOCTYPE

   The DOCTYPE of the document MUST be "html", which declares that the
   document is compliant with HTML5.  The document will start with
   exactly this string:

   <!DOCTYPE html'>

   The SYSTEM 'about:legacy-compat' portion MAY be dropped in the future
   if the tooling chosen to produce this format does not require it.

3.2.3.  Root Element

   The root element of the document MUST be <html>.  This element SHOULD
   include a lang attribute, whose value is a RFC5646 language tag
   describing the natural language of the document.  For documents
   submitted to the RFC Series or Internet-Draft Series, the language
   tag MUST be 'en', meaning "English".  If the lang attribute is not
   present, its value should be taken to be 'en'.

3.2.4.  Charset Declaration

   In order to be correctly processed by browsers that load the HTML
   using a mechanism that does not provide a valid MIME content-type or
   charset, the HTML <head> element MUST contain a <meta> element, with
   charset attribute with value 'utf-8'.  This will look like:

   <meta charset='utf-8' >

3.2.5.  Style

   The <head> SHOULD contain an embedded CSS stylesheet in a <style>
   element.  The styles in the stylesheet are to be set consistently
   between documents by the RFC Editor, according to the best practices
   of the day.  The RFC Editor SHALL choose a stylesheet that does not
   modify the meaning of the normative text of the document.  The RFC
   Editor SHALL make the stylesheet available via a standard protocol
   (e.g.  HTTP or HTTPS) for ease of authorship.  However, when a
   document is submitted, external stylesheets (other than "local.css"

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   as specified below) are NOT ALLOWED.  The stylesheet itself MUST NOT
   be considered as normative information.

   To ensure consistent formatting, individual style attributes SHOULD
   NOT be used in the main portion of the document source except in
   highly exceptional circumstances; each use MUST be individually

   Different readers of a specification will desire different tweaks to
   the stylesheet.  To facilitate this, the <head> SHOULD include a
   <link> to a stylesheet in the same directory as the HTML file, named
   "local.css", after the embedded stylesheet.  Note that this
   "local.css" file will not exist for most users; browsers will
   correspondingly ignore this <link>.

   For example:

       <style type='text/css'>
   <! --
       /* RFC-editor styles */
       <link rel='stylesheet' type='text/css' href='local.css' />

3.2.6.  Emphasis

   Words or phrases may be emphasized using the <em> element, usually
   rendered as italics.  Strong emphasis may be donated with the
   <strong> element, which is usually rendered as boldface.  Underlining
   MUST NOT be used except for links, to avoid visual confusion.  Text-
   only emphasis such as "_bold_" MUST NOT be used.

   The RFC Editor will set a policy that reflects the current feelings
   of the community as to whether this emphasis markup is allowed in
   documents that are submitted for publication in the RFC series.


   HTML comments will not be generated by the rendering agent from the
   canonical XML.

3.2.8.  Sections

   Each section of the document SHALL be formatted as a <div> tag, with
   a class attribute with value 'section'.  A document-unique id
   attribute SHOULD be assigned to each section <div>.

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   NOTE: HTML5 requires id attributes to be unique across an entire

   Each section <div> MUST contain a header tag (<h2>-<h6>) of the
   appropriate depth, with top-level sections getting an <h2> tag, and
   each nested section getting the next higher header level.  If more
   than five levels of headers are required, <h6> MUST be used for each
   deeper-nested section.  However, nesting sections more than five
   levels deep is NOT RECOMMENDED.

   The text in each header tag MUST begin with the section number.
   Section numbers MUST begin at "1.", and MUST increment by one for
   each successive section at the same level.  Subsections MUST be
   numbered by appending the subsection number to the parent section

   It is RECOMMENDED that the section number be wrapped in an <a>
   element, whose href attribute points to the corresponding section div
   with a local relative reference.  This <a> element SHOULD have the
   CSS class self-ref.

   Within a section, each "normal" paragraph MUST be surrounded by a <p>

   For example:

   <div class='section' id='example'>
     <h2><a class='self-ref' href='#example'>1.</a>Example Section
     <p>This is a description of the example</p>
     <div class='section' id='nested'>
       <h3><a class='self-ref' href='#nested'>1.1.</a> Nested Section
       <p>This is a description of the nested section.</p>
       <p>This is the second description paragraph.</p>

   Parent sections that contain child sections MUST NOT contain "normal"
   paragraphs after a sub-section.  For example, the following is

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   <div class='section' id='example'>
     <h2><a class='self-ref' href='#example'>1.</a> Example Section
     <p>This is a description of the example</p>
     <div class='section' id='nested'>
       <h3><a class='self-ref' href='#nested'>1.1.</a>Nested Section
       <p>This is a description of the nested section.</p>
     <p>BAD PROSE!</p>

3.2.9.  Appendices

   Appendices are special cases of top-level sections.  Each appendix of
   the document SHALL be formatted as a <div> tag, with a class
   attribute with value 'appendix'.  A document-unique, id attribute
   SHOULD be assigned to each section <div>.  The id MAY be human-
   readable or generated.  Each appendix <div> MUST contain an <h2>
   element containing text that describes the purpose of the appendix.
   Appendices are identified to the reader with Latin capital letters
   A-Z, in order.  It is NOT RECOMMENDED to have more than 26
   appendices, but if required, appendices "AA.", "AB.", etc. follow
   Appendix Z.

   Inside the appendix, subsections MUST be formatted per Sections,
   numbered sequentially.  For example, the first subsection of
   "Appendix A." is "Appendix A.1.".

   For example:

   <div class='appendix' id='acknowledgements'>
     <h2>Appendix A. Acknowledgements</h2>
     <p>The author gratefully acknowledges the contributions of...</p>
     <div class='section' id='contributors'>
       <h3>Appendix A.1. Contributors</h2>
       <p>These people contributed text...</p>

3.2.10.  Paragraphs

   Paragraphs MUST be contained in a section <div> or an appendix <div>.
   A document-unique, id attribute SHOULD be assigned to each <p>.  The
   id will usually be machine-generated, but MAY be human-readable if

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   It is RECOMMENDED that each paragraph be kept relatively small
   compared to a "page" in previous RFC formats, so that references to
   each paragraph are at least as valuable as page references have been
   in previous formats.

3.2.11.  Lists

   Lists may be used inside a section <div>, and may nest in other lists
   as needed.  However, lists MUST NOT be nested inside a <p> element.
   Unordered lists (<ul>) and ordered lists (<ol>) may both be used.
   For example:

   <div class='section' id='lists'>
     <h4>Unordered list</h4>
     <p id='lists-p-1'>An explanation:</p>
         <li>Two.1: (this one is numbered)</li>

3.2.12.  References

   Reference format must follow the guidance in the RFC Style Guide


   Non-trivial direct quotes from other documents SHOULD use the
   <blockquote> element.  If the quote needs a citation, wrap the
   <blockquote> in a <figure> and add a <figcaption> element that
   contains text (and possibly links) that describe the quote.  For
   example, this code:

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       <p>Here a <code>blockquote</code> element
       is used in conjunction with a <code>figure</code> element and
       its <code>figcaption</code> to clearly relate a quote to its
       attribution (which is not part of the quote and therefore
       doesn't belong
       inside the <code>blockquote</code> itself):</p>
     <figcaption>Sample Quote from <a class='ref' href='#w3c:
       section 4.5.4</figcaption>

   Would render as:

       Here a blockquote element is used in conjunction with a figure
       element and its figcaption to clearly relate a quote to its
       attribution (which is not part of the quote and therefore
       doesn't belong inside the blockquote itself):

             Figure 1: Sample Quote from HTML5, section 4.5.4

3.3.  More Elaborate Information

   This section describes how to format several types of information
   that occur regularly in documents for the Internet-Draft and RFC
   Series which are not descriptive text.

3.3.1.  Requirement Keywords

   The RFC2119 keywords in the document will be set off with special
   markup.  They MUST be surrounded with a <span> element containing the
   CSS class rfc2119.  For example:

   They <span class='rfc2119'>MUST</span> be surrounded

3.3.2.  Formatting the Table of Contents

   The table of contents for the document MUST appear in a <div>
   element, which SHOULD precede any of the sections of proper document
   content.  The <div> element MUST have an id attribute with value
   'toa'.  The <div> element SHOULD contain an <h2> element containing
   the string Table of Contents, followed by nested <ul> and <li>
   elements describing the structure of the document, with links to each
   of the sections mentioned.  For example:

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   <div id='toc'>
     <h2>Table of Contents</h2>
         <div>1. <a href='#introduction'>Introduction</a></div>
             <div>1.1. <a href='#background'>Background</a></div>

   NOTE: the Table of Contents SHOULD NOT be considered meta-data for
   the document.  The sections themselves SHOULD contain all of the data
   that is required.

3.3.3.  SVG

   SVG can be included directly in the HTML source, surrounded by a
   <figure> element and succeeded by a <figcaption> element, as
   described in Section .  The root <svg> element MUST contain a <title>
   or <desc> element that fully describes the diagram for accessibility
   to screen readers; this is similar to the alt; attribute on images.
   See "SVG Drawings for RFCs: SVG 1.2 RFC" for details on the
   appropriate SVG profile for use in RFCs [I-D.brownlee-svg-rfc].

3.3.4.  Inline Code

   Use the <code> element to set aside literal references to code or
   protocol elements in the middle of a paragraph.  If desired, the
   language of the code or protocol can be declared using a class
   attribute starting with language-. For example:

   Use the <code class='language-html'><;code>;</code> element

3.3.5.  Blocks of Code

   Larger sections of code or protocol can be included using a <pre>
   element with a class attribute of code.  If desired, the language of
   the code or protocol can be declared using a further class value
   starting with 'language-' (multiple class values are separated by
   spaces in HTML).  The text inside the <pre> element will be rendered
   in a monospace font, with whitespace maintained.  For example:

   <pre class='code language-html'>
     &lt; /&gt;

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   Will be rendered as:

     < />

   Depending on author style, blocks of code MAY be enclosed in a
   <figure> element, with a <figcaption> element that describes the
   block.  For example, see Figure 2.

   <figure id='blockfigure'>
   <pre class='code language-html'></pre>
   <figcaption>A code block wrapped in a figure.</figcaption>

                Figure 2: A code block wrapped in a figure

3.3.6.  ASCII Art

   ASCII art is still preferred by some authors in preference to an
   image or SVG.  The RFC Editor may decide to prefer SVG, or may decide
   to prohibit ASCII art in the future, depending on the needs of the
   community at the time of publishing.  Until that time, to include
   ASCII art, wrap a <pre> element with class='ascii' in a <figure>
   along with a <figcaption>, as if the <pre> element were an
   Section 3.3.4 image.  For example:

     <pre class='ascii'>

                   | original  | &lt;+
                   +-----------+  |
                     |            |
                     | nit        | edit
                     v            |
     nit (no-op)   +-----------+  |
   +-------------- |           |  |
   |               | canonical |  |
   +-------------&gt; |           | -+

     <figcaption>Sample ASCII art</figcaption>

                        Figure 3: Sample ASCII art

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3.3.7.  Packet Formats

   Packet format descriptions can be encoded as a <table> element
   wrapped in a <figure> along with a <figcaption>, as if the <pre>
   element were an image, as specified in Section .  For consistent
   formatting, the <table> element should have class pdu.  For example:

     <figcaption>Sample packet format</figcaption>
     <table class='pdu'> [table describing the packet] </table>

4.  Document Metadata

   Metadata for the document SHOULD be easily extractable from the
   document by tools that ordinarily process HTML.  Typically, the class
   and id attributes can be used to query the document using CSS-style
   selectors.  The metadata scheme SHOULD be designed such that the
   element name is not required in order to select a given piece of
   data.  Instead, any element that can contain text can be used for a
   given class or id to be selected.  The value of the data contained by
   the selected element(s) consists of the concatenation of all of the
   text from all of the child nodes of the selected element or elements,
   with each run of consecutive whitespace Unicode codepoints
   [codepoints with the White_Space property, such as U+0020 (SPACE),
   U+2029 (PARAGRAPH SEPARATOR)] compressed to a single U+0020 (SPACE).
   The metadata scheme MUST allow unambiguous selection.

   The id attribute is used to identify pieces of data that are
   guaranteed to be unique across the document.  Any element with an id
   attribute can also be used as a fragment target in a URI by starting
   with the base URI of the document, then appending "#" (U+0023: NUMBER
   SIGN) and the value of the id attribute.  In CSS, the element with a
   given id attribute value is selected by prepending the value with '#'
   (U+0023: NUMBER SIGN).  For example, the following HTML in a document
   with the URI http://example.com/index.html:

   <div id='example'>Important Text</div>

   Can be targeted directly with the URL http://example.com/
   index.html#example, and the CSS selector #example.

   The class attribute is a catch-all tagging mechanism for everything
   in the document that might not be unique.  Multiple classes may be
   defined on a single element by setting the class attribute to a
   space-separated list of classes.  All of the elements with a given

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   class name can be selected in CSS by prepending the class name with
   "."  (U+002E: FULL STOP).

4.1.  Document Information

   Information about the document as a whole.  The <div> element with
   id='document' SHOULD be the first child element of the HTML.  For

   <div id='document'>
     <div class='identifiers'>
       <div class='workgroup'>Network Working Group</div>
       <div class='series'>Internet-Draft</div>
       <div class='status'>Standards Track</div>
       <div class='published'>2012-07-07</div>
       <div class='expires'>2013-01-07</div>
       <div class='version'>03</div>
     <div class='authors'>
       <div class='author'>
         <span class='initial'>J.</span>
         <span class='surname'>Hildebrand</span>
         <span class='company'>Cisco Systems, Inc.</span>

   More details for this format will be included in future drafts of
   this document.

4.2.  Title

   The title of the document MUST appear in an <h1> element, which
   SHOULD follow directly after the Document Information.  The <h1>
   element MUST have an id attribute with value 'title'.  For example:

   <h1 id='title'>HTML RFC Format</h1>

4.3.  Abstract

   The abstract for the document MUST appear in a <div> element, which
   SHOULD follow directly after the Title.  The <div> element MUST have
   an id attribute with value 'abstract'.  The <div> element SHOULD
   contain an <h2> element containing the word Abstract, and MUST
   contain one or more <p> elements containing text that describes the
   document succinctly.  For example:

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   <div id='abstract'>
     <p>This document defines an HTML format...</p>

4.4.  IPR Statements

   The IPR boilerplate for the document MUST appear in a <div> element,
   which SHOULD follow directly after the Abstract.  The <div> element
   MUST have an id attribute with value 'ipr' and a CSS class of the
   name of the relevant IPR ruleset.  The only valid values for the IPR
   ruleset class are trust200902, noModificationTrust200902, and
   noDerivativesTrust200902 at this time.  The contents of the <div>
   element are to be set correctly for the given ruleset, based on
   guidance from the IETF trust.  For example:

   <div id='ipr' class='trust200902'>
     <h2>Status of this Memo</h2>
     <h2>Copyright Notice</h2>

4.5.  Author

   This section will be augmented with normative text when an approach
   is decided upon.  A quick example (as an existence proof) can be
   found in Figure 4.

   <address class='vcard'>
     <span class='n hidden'>
       <span class='family-name'>Hildebrand</span>
       <span class='given-name'>Joe</span>
     <span class='nickname hidden'>hildjj</span>
     <span class='fn'>Joe Hildebrand</span>
     <span class='org'>Cisco Systems, Inc.</span>
     <a class='email' href='jhildebr@cisco.com'>jhildebr@cisco.com</a>
     <div class='adr'>
       <div class='street-address'>1899 Wynkoop St, Suite 600</div>
       <div><span class='locality'>Denver</span>,
            <span class='region'>CO</span>
            <span class='postal-code'>80202<span></div>
       <div class='country-name'>United States</div>

              Figure 4: Sample (temporary) author information

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4.6.  Bibliographical Information


5.  Examples

5.1.  Self

   This draft itself is a good example of how to use the format.  Please

5.2.  Code Sample

   #include \<stdio.h\>
   int main(int argc, char **argv)
       printf("Hello, IETF\n");
       return 0;

5.3.  Sequence Diagrams

   Include an image tag with class='sequence', where the alt; text is
   the WebSequenceDiagrams.com source for the diagram.

   Before publication, this approach will be replaced by something more
   well-specified and not requiring third-party software.

     <img class='sequence' alt;='
   title Authentication Sequence
   Alice->;Bob: Authentication Request
   note right of Bob: Bob thinks about it
   Bob->;Alice: Authentication Response' >
     <figcaption>A sample sequence diagram</figcaption>

   title Authentication Sequence Alice->Bob: Authentication
   Request note right of Bob: Bob thinks about it Bob->Alice:
   Authentication Response

                    Figure 5: A sample sequence diagram

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5.4.  ABNF

   Augmented Backus-Naur Form is a way of describing formal syntax,
   described in RFC5234.  Include ABNF (without extra indentation) in a
   <pre> element, with CSS class 'code' and "language-abnf".  For

   <pre class='code language-abnf'>
   label        = top-level *4section-num
   top-level    = section-num / appendix-let
   section-num  = 1*DIGIT "."
   appendix-let = 1*CAP "."
   CAP          = %x41-5A ; A-Z
   DIGIT        = %x30-39 ; 0-9

   Is rendered as:

   label = top-level _4section-num top-level = section-num / appendix-
   let section-num = 1_DIGIT "." appendix-let = 1*CAP "."  CAP = %x41-5A
   ; A-Z DIGIT = %x30-39 ; 0-9

6.  Security Considerations

   Since RFCs are sometimes exchanged outside the normal Web sandboxing
   mechanism (e.g. rsync to a mirror) then loaded from a local file,
   more care must be taken with the HTML than is ordinary on the Web. In
   particular, the intent with the format is to forbid any embedded code
   such as JavaScript as well as all mechanisms that could be used to
   execute code outside of the browser such as plugins or non-static
   media (such as video).

7.  IANA Considerations


8.  Appendix A.  Allowable Subset of HTML

   This section collects all of the elements that are allowed in the
   HTML RFC format.  Each element is listed with a set of allowed
   attributes, and a list of the parent elements in which the element
   may be placed.  The attributes class, id, and lang are allowed on all
   elements.  All other elements, attributes, and nesting approaches
   MUST NOT be used.

   | Element    | Attributes                             | Parents     |

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   | a          | href, title                            | address,    |
   |            |                                        | div,        |
   |            |                                        | figcaption, |
   |            |                                        | h2, h3, h4, |
   |            |                                        | h5, li, p,  |
   |            |                                        | span, td    |
   |            |                                        |             |
   | address    |                                        | div         |
   |            |                                        |             |
   | blockquote |                                        | figure      |
   |            |                                        |             |
   |            |                                        | html        |
   |            |                                        |             |
   | br         |                                        | td, th      |
   |            |                                        |             |
   | code       |                                        | blockquote, |
   |            |                                        | li, p, td   |
   |            |                                        |             |
   | div        |                                        | address, ,  |
   |            |                                        | div, li     |
   |            |                                        |             |
   | em         |                                        | p, span     |
   |            |                                        |             |
   | figcaption |                                        | figure      |
   |            |                                        |             |
   | figure     |                                        | div         |
   |            |                                        |             |
   | h1         |                                        |             |
   |            |                                        |             |
   | h2         |                                        | div         |
   |            |                                        |             |
   | h3         |                                        | div         |
   |            |                                        |             |
   | h4         |                                        | div         |
   |            |                                        |             |
   | h5         |                                        | div         |
   |            |                                        |             |
   | head       |                                        | html        |
   |            |                                        |             |
   | html       |                                        |             |
   |            |                                        |             |
   | img        | alt;, height, src, width               | figure      |
   |            |                                        |             |
   | li         | 1, 1.0, 1.1, 10, 10646-1, 16, 2, 2026, | ol, ul      |
   |            | 2026., 206, 2119., 2418., 2739., 3.2,  |             |
   |            | 329, 3978, 4, 4748, 495, 617, 79, a,   |             |
   |            | abbrev, abnf, abnf., about,            |             |
   |            | additional, all, alpha, also, alt;o,   |             |

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   |            | among, an, analyzer, and, any,         |             |
   |            | applications, are, as, at, audio,      |             |
   |            | augmented, authors, available, backus- |             |
   |            | naur, balances, bcp, be, been,         |             |
   |            | beginning, benefit, best, between,     |             |
   |            | bnf, both, box, but, by, called, can,  |             |
   |            | capitalized., cases, channel.,         |             |
   |            | character, characteristic, checking,   |             |
   |            | chromaticity, color, common,           |             |
   |            | communities, community, compactness,   |             |
   |            | compatibility, compatible, concerning, |             |
   |            | constraints, contributions,            |             |
   |            | contributors, core, coyote, creation,  |             |
   |            | current, data, data-xml, day, define,  |             |
   |            | defines, definitions, delivery,        |             |
   |            | depths, described, describes,          |             |
   |            | designed, desirable, details,          |             |
   |            | detection, developed, development,     |             |
   |            | differences, discussion, display,      |             |
   |            | document, document:, documents,        |             |
   |            | documents., email, encoding,           |             |
   |            | encodings, encompasses, ensure,        |             |
   |            | entities, errors., exchanging,         |             |
   |            | extensible, extensions, file, follow,  |             |
   |            | for, force, form, formal, format,      |             |
   |            | formatted, from, full, fullname,       |             |
   |            | fully, gamma, gif, graphics, groups,   |             |
   |            | guidelines, has, have, heterogeneous,  |             |
   |            | hill, holders., how, identifying, iec, |             |
   |            | ietf, ietf., image, images, images.,   |             |
   |            | improved, in, including, incorporate,  |             |
   |            | indicate, individuals, information,    |             |
   |            | initials, integrity, intellectual,     |             |
   |            | intended, interchange., internet,      |             |
   |            | interpreted, involve, ipr, is, iso,    |             |
   |            | it, key, language, large, led,         |             |
   |            | legitimate, level, lexical, made,      |             |
   |            | many, markup, mass., matching, may,    |             |
   |            | media, meet., memo, memo., modified,   |             |
   |            | month, most, much, mult;iple, must,    |             |
   |            | name, naming, near, need, network,     |             |
   |            | not, note, object, object.,            |             |
   |            | objectives, obsoletes, octets, of,     |             |
   |            | often, on, online, option., optional,  |             |
   |            | originally, other, over, palo,         |             |
   |            | paragraph, parsers, participants,      |             |
   |            | patent, patent-free, permitting,       |             |
   |            | phrase, plus, png, policies, popular,  |             |

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   |            | possible., power., practices,          |             |
   |            | preserving, private, progressive,      |             |
   |            | property, proposal, proposed, provide, |             |
   |            | provides, providing, public, range,    |             |
   |            | ranges., raster, reasonable,           |             |
   |            | recommended, reference, register,      |             |
   |            | related, relative, rely, replace,      |             |
   |            | replacement, replaces,                 |             |
   |            | representational, representing,        |             |
   |            | requests, required, requirement,       |             |
   |            | requirements, research, respecting,    |             |
   |            | retain, revision, rfc, rfc2083,        |             |
   |            | rfc2119, rfc2397, rfc3629, rfc3979,    |             |
   |            | rfc5234, rfc5378, rfc5646, rfc6350,    |             |
   |            | rfcs, rights, rule, s, sample,         |             |
   |            | section, semantics, set, several,      |             |
   |            | shall, sheets, should, signify,        |             |
   |            | simple, simplicity, so, software,      |             |
   |            | specification, specification.,         |             |
   |            | specifications, specifications.,       |             |
   |            | specifications:, specifies, standard,  |             |
   |            | standards, storage, store, streamable, |             |
   |            | structured, style, such, suggestions,  |             |
   |            | supplies, surname, syntax, syntax.,    |             |
   |            | systems., tags, target, technical,     |             |
   |            | technologies, technology, telephone,   |             |
   |            | that, the, their, these, they, this,   |             |
   |            | tiff., to, track, transformation,      |             |
   |            | transmission, transparent, truecolor,  |             |
   |            | type, universal, updates, url, us-     |             |
   |            | ascii, use, used, used., user-defined, |             |
   |            | uses, utf-8, value, values, variety,   |             |
   |            | vcard, vector, version, viewing, well, |             |
   |            | well-compressed, were, where, which,   |             |
   |            | while, who, wide, with, within, words, |             |
   |            | work, worked, working, world, writing, |             |
   |            | year                                   |             |
   |            |                                        |             |
   | meta       | charset, content, name                 | head        |
   |            |                                        |             |
   | ol         |                                        | div         |
   |            |                                        |             |
   | p          |                                        | div, li, td |
   |            |                                        |             |
   | pre        |                                        | div, figure |
   |            |                                        |             |
   | span       |                                        | address,    |
   |            |                                        | div, li, p, |

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   |            |                                        | span        |
   |            |                                        |             |
   | strong     |                                        | p, pre      |
   |            |                                        |             |
   | svg        | height, viewbox, width                 | figure      |
   |            |                                        |             |
   | table      |                                        | div, figure |
   |            |                                        |             |
   | t          |                                        | table       |
   |            |                                        |             |
   | td         | colspan                                | tr          |
   |            |                                        |             |
   | th         | colspan                                | tr          |
   |            |                                        |             |
   | thead      |                                        | table       |
   |            |                                        |             |
   | title      |                                        | head        |
   |            |                                        |             |
   | tr         |                                        | t, thead    |
   |            |                                        |             |
   | ul         |                                        | div, li,    |
   |            |                                        | td, ul      |

9.  Appendix B.  CSS Classes with Special Meaning

   Although the author can add class information to any element, the
   following class names have special meaning in an HTML RFC:

                      | Class            | Meaning |
                      | adr              |         |
                      |                  |         |
                      | appendix         |         |
                      |                  |         |
                      | ascii            |         |
                      |                  |         |
                      | author           |         |
                      |                  |         |
                      | authors          |         |
                      |                  |         |
                      | code             |         |
                      |                  |         |
                      | company          |         |
                      |                  |         |
                      | country-name     |         |
                      |                  |         |

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                      | date             |         |
                      |                  |         |
                      | docName          |         |
                      |                  |         |
                      | edge             |         |
                      |                  |         |
                      | email            |         |
                      |                  |         |
                      | expires          |         |
                      |                  |         |
                      | family-name      |         |
                      |                  |         |
                      | figref           |         |
                      |                  |         |
                      | fn               |         |
                      |                  |         |
                      | given-name       |         |
                      |                  |         |
                      | graph            |         |
                      |                  |         |
                      | hidden           |         |
                      |                  |         |
                      | identifiers      |         |
                      |                  |         |
                      | initial          |         |
                      |                  |         |
                      | initials         |         |
                      |                  |         |
                      | invalid          |         |
                      |                  |         |
                      | languag-hmtl     |         |
                      |                  |         |
                      | language-abnf    |         |
                      |                  |         |
                      | language-c       |         |
                      |                  |         |
                      | language-html    |         |
                      |                  |         |
                      | locality         |         |
                      |                  |         |
                      | n                |         |
                      |                  |         |
                      | nickname         |         |
                      |                  |         |
                      | node             |         |
                      |                  |         |
                      | note             |         |
                      |                  |         |

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                      | org              |         |
                      |                  |         |
                      | pdu              |         |
                      |                  |         |
                      | postal-code      |         |
                      |                  |         |
                      | published        |         |
                      |                  |         |
                      | ref              |         |
                      |                  |         |
                      | reflinks         |         |
                      |                  |         |
                      | region           |         |
                      |                  |         |
                      | rfc2119          |         |
                      |                  |         |
                      | rfceditor-remove |         |
                      |                  |         |
                      | section          |         |
                      |                  |         |
                      | sectref          |         |
                      |                  |         |
                      | self-ref         |         |
                      |                  |         |
                      | sequence         |         |
                      |                  |         |
                      | series           |         |
                      |                  |         |
                      | series-info      |         |
                      |                  |         |
                      | status           |         |
                      |                  |         |
                      | street-address   |         |
                      |                  |         |
                      | surname          |         |
                      |                  |         |
                      | title            |         |
                      |                  |         |
                      | toc              |         |
                      |                  |         |
                      | todo             |         |
                      |                  |         |
                      | trust200902      |         |
                      |                  |         |
                      | vcard            |         |
                      |                  |         |
                      | version          |         |
                      |                  |         |

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                      | workgroup        |         |

10.  Appendix C.  Element IDs with Special Meaning

   Although the author can add an id attribute to any element, the
   following id values SHOULD NOT be used except for the role defined
   for each below:

   ID Meaning document Data about the document, including dates, name,
   version, etc.

   title The title of the document, usually applied to a <h1> element.

   abstract The abstract for the document, usually applied to a <div>
   element that contains a heading and paragraphs of text.  ipr

   The Intellectual Property Rights associated with the document.  The
   class attribute of the same element will contain a machine-readable
   IPR statement name from this list:

   o  trust200902: This is appropriate for most drafts, where the entire
      content of the draft is written by the draft's authors, or all
      authors of other material have given explicit permission to use
      their work.

   o  noModificationTrust200902: This is appropriate for drafts where
      the authors wish to place the additional condition that if the
      draft is published as an RFC, it must have no changes other than
      formatting.  An example might be a document published by another
      organization that permits copying but not modification.

   o  noDerivativesTrust200902: This is appropriate for drafts not
      intended to be published as RFCs.

   o  pre5378Trust200902: This is appropriate for drafts that include
      material submitted to the IETF prior to RFC 5378 (10 Nov 2008),
      where the authors of that material have not given explicit
      permission to use their work in this draft.  An example might be a
      draft using material from an RFC whose author has died or cannot
      be located, or who thinks your draft is stupid.

   The element with this id will contain all of the IPR and status
   boilerplate text

   Note: an IANA registry may be required for this attribute in the

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   venue The venue for discussion.  Inside the element tagged with this
   id will be one or more <a> elements that describe the discussion
   venue for Internet-Drafts.

   toc The Table of Contents

   references The section containing bibliographical data, including
   sections for normative and informative references.

   normative The section containing normative document references.

   informative The section containing informative document references.

   authors The section containing data about the authors of the

   security The section containing the Security Considerations for the

   iana The section containing the IANA Considerations for the document.

   acknowledgments The section containing the author's acknowledgments.

11.  Acknowledgments

   The author gratefully acknowledges the contributions of: Patrick

12.  References

12.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

              Berjon, R., Faulkner, S., Leithead, T., Navara, E.,
              O&#039;Connor, E., and S. Pfeiffer, "HTML5", World Wide
              Web Consortium CR CR-html5-20130806, August 2013,

              Bos, B., A&#135;elik, T., Hickson, I., and H. Lie,
              "Cascading Style Sheets Level 2 Revision 1 (CSS 2.1)
              Specification", World Wide Web Consortium Recommendation
              REC-CSS2-20110607, June 2011,

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12.2.  Informative References

              Brownlee, N., "SVG Drawings for RFCs: SVG 1.2 RFC", draft-
              brownlee-svg-rfc-04 (work in progress), April 2014.

              Flanagan, H., "RFC Style Guide", draft-iab-styleguide-02
              (work in progress), April 2014.

              Hoffman, P., "The 'XML2RFC' version 3 Vocabulary", draft-
              hoffman-xml2rfc-08 (work in progress), May 2014.

Authors' Addresses

   Joe Hildebrand
   Cisco Systems, Inc.

   Email: jhildebr@cisco.com

   Heather Flanagan
   RFC Editor

   Email: rse@rfc-editor.org

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